The travel surface can be a railway or traffic road, although the invention is likewise applicable to other travel surfaces such as runways of an airport, wherein the air traffic causes lateral emission of aircraft noise. Different options are known for limiting, at least for determined frequency ranges, the lateral emission of sound originating from sound sources travelling over a railway, traffic route or runway (motor vehicles such as cars, trucks, motorbikes, trains and the like). A first option is to place a noise-reducing screen or a noise barrier along the travel surface. The sound coming from the sound source (i.e. sources originating from motorized road traffic or a train) is reflected and/or absorbed by the noise-reducing screen, whereby a low-noise zone is created behind the noise-reducing screen. The sound level at ground surface level or thereabove is therefore generally lower behind the noise-reducing screen (as seen from the travel surface) than in front of the noise-reducing screen.
Such noise-reducing screens or noise barriers are however expensive provisions, may be perceived as unattractive and often require complex constructions, particularly in respect of the foundation, in light of the high forces which are exerted on noise-reducing screens as a result of wind. The noise-reducing screens or noise barriers further obstruct the view of the surrounding area for the traffic participant, which can be perceived as disagreeable.
The noise of the traffic is determined by a number of different sound sources. In the case of motorized road traffic there are sources such as the engine, the tyres (rolling noise of tyres over the roadway, dominant above a speed of 30 km/hour) and the noise caused by the flow of the air round the vehicle. Similar sound sources can be identified in the case of rail traffic. These sound sources are usually located relatively close to the ground (i.e. the travel surface), characteristically at a distance of less than a meter therefrom. Use is made hereof in an alternative to the above stated noise-reducing screens or noise barriers. In the document WO 2011 049454 A2, the content of which should be deemed as incorporated herein, the lateral emission of sound is prevented by a number of resonators arranged parallel along the travel surface. These resonators are not configured to cause sound absorption but provide for an effective diffraction of the sound incident in substantially shearing manner from the sound sources. The resonators create a diffracting effect which depends on the associated resonance frequency of the air in the resonator. This resonance frequency depends on, among other factors, the form and dimensions (i.e. the dimensioning) of the relevant resonator. In addition, the resonance frequency of a resonator depends on the dimensions of the resonators located nearby.
The sound in a determined frequency range can be diffracted in upward direction when resonators with different resonance frequencies are applied. This diffraction is of course dependent on the frequency. Since the most dominant tones in traffic noise generally lie in a limited frequency range, for instance from 800 Hz to 1200 Hz, a suitable diffraction can be realized in the relevant frequency range with a correct dimensioning and positioning of the resonators.
A noise reduction takes place at a determined angle relative to the horizontal, up to about 30° to 40°, in that the sound is effectively diffracted upward, i.e. above said determined angle. This effect takes place in a lateral direction (relative to the travel surface, i.e. perpendicularly of the longitudinal direction of the travel surface). The closer the resonators are disposed to the sources, the greater the angle to be realized becomes within which a considerable noise reduction can be realized.
Because the resonators can be placed relatively close to the sound sources, the ‘screening’ effect of resonators can be said to be considerable. The surrounding area, i.e. particularly the neighbourhood with for instance houses behind the diffractors, as seen from the travel surface, for which the angle relative to the horizontal will amount in practice to no more than a few degrees (depending on the distance between the travel surface and buildings), will generally be exposed to a greatly reduced level of traffic noise. The resonators are further arranged in the ground along the travel surface, or form part thereof. Because they are located very close to the ground they are less of a problem from a visual viewpoint, and substantially lower forces resulting from wind load are exerted.
The known resonators do however have a number of drawbacks.
A first drawback is that rainwater or other liquids may penetrate the resonators. If this is the case, the diffracting action of the resonators, and thereby the effective sound attenuation, decreases immediately. The rainwater can get into the resonators directly in the form of rain, but can also be the result of water being splashed from the surface of the roadway. The intermediate walls between adjacent resonators are closed and ensure that rainwater which has penetrated a resonator also remains contained in the resonator. In order to nevertheless enable drainage of rainwater the above stated publication WO 2011 049454 A2 proposes arranging in the bottom of the resonators drainage channels along which the water can be drained. As shown more specifically in FIG. 7 of this publication, drainage pipes are connected to openings in the bottom of the resonators. Surprisingly however, it has been found that these drainage channels can in some cases have a negative effect on the operation of the resonators such that a reduced sound attenuation is realized.
A further drawback of the known resonators is that they have a length such that traffic nevertheless travelling over the resonators, particularly two-wheeled vehicles such as cycles or motorbikes, may be adversely affected thereby. The front or rear tyre of such vehicles may find their way into the resonators, which can result in dangerous situations.